Railing Length

Calculate The Stair Railing Length

Let Pythagoras Be Your Friend

In the drawing above, the rise is the distance from the surface of your deck to the ground. The run is roughly the distance from the spot on the ground where your railing post will be anchored to the spot on the ground under where your top railing post will be anchored. That's a little different measurement then the rise & run in the angle calculation above. The reason is that the length of the railing depends on where you set your posts.

Stair Railing

Remember in high school when you slept through geometry on the premise that you would never use it? Well, think again. Calculating the length of your stair railing actually is actually based on a fundamental relationship in geometry called the Pythagorean Theorem, which basically states (in terms of railing) that the squared length of your stair railing is equal to the squared rise of your stairs + the squared run of your stairs, or A^2 + B^2 = C^2.

For example, suppose your rise & run (A & B in the drawing above, respectively) are 40" & 60".

Then: 40^2 + 60^2 = C^2

working through this problem gives us the following steps:

1) 1600 + 3600 = C^2
2) 5200 = C^2
3) C = the square root of 5200
4) C = 72.11 inches

So the length of stair railing that you need is 72.11 inches. Which brings us to another important point. Since most outdoor stair railings come in whole sizes like 3ft., 4ft., 5ft., etc. what do you do when your measurement is between 2 sizes? The answer is that you probably need to round up to the next size. In this case though, .11 inches isn't very much and we could probably move our posts closer together by that much and stick with the 72" or 6' railing.